Adblue is the trade name of an aqueous urea solution which, by weight, consists of 32.5 Mass.-% consists of pure urea and demineralized water. The colorless, odorless and nontoxic water-urea mixture is used to reduce nitrogen oxide (NOx) emissions from diesel engines in vehicles. The trademark rights to the name AdBlue are held by the German Association of the Automotive Industry e.V. (VDA). The water-urea mixture is referred to as AUS (Aqueous Urea Solution) 32 in the international standard ISO 22241 (comparable to the German standard DIN 70070) and is also known in South America by the abbreviation Arla 32. In North America, the urea solution is sold under the name Diesel Exhaust Fluid (DEF). Skin contact with the liquid, which is not a hazardous substance, should generally be avoided, but is not dangerous.
Nitrogen oxide emissions can only be reduced with AdBlue in conjunction with an SCR catalytic converter (Selective Catalytic Reduction). The water-urea solution is injected into the exhaust channel upstream of the SCR catalytic converter at high pressure with the aid of a metering pump. Here, AdBlue is converted to ammonia via an intermediate step. Only with the released ammonia can the nitrogen oxides in the SCR catalytic converter coated with vanadium, tungsten or titanium oxide – at temperatures from approx. 170°C to nitrogen. Water shall be implemented. The optimum AdBlue quantity for this reaction depends on the nitrogen oxide emission of the engine, which varies with the engine speed and the required torque. In practice, AdBlue quantities are dosed via a previously determined engine map.
SCR technology can reduce nitrogen oxide emissions by up to 90%. The consumption of AdBlue amounts to between 2% and 8% of the diesel fuel consumption and is estimated by the VDA at an average of approx. 1.5 liters per 1000 kilometers indicated. By optimizing engine management, trucks can reduce their fuel consumption by up to 6% with the help of AdBlue.
The water-urea solution requires an additional tank, which must be made of plastic or stainless steel, as AdBlue has a corrosive effect on most metals. While such an AdBlue tank holds about 17 liters for most cars, it is much larger for trucks. Since AdBlue from ca. -11°C, the tanks are also equipped with an additional heater for use in regions where severe frost is expected.
Adblue is produced from a mixture of natural gas and nitrogen, from which ammonia is first produced using the Haber-Bosch process. With added carbon dioxide, the toxic and highly irritating gas ammonia is then turned into a liquid urea melt in the urea synthesis reaction. It is brought to the correct percentage ratio by adding demineralized water in the final production step. In-house production with solid urea is not recommended, as exceeding the required purity level can damage the SCR catalytic converter.
While AdBlue and SCR catalysts have been used for trucks for some time to enable them to meet the Euro 4 and Euro 5 emissions standards, the technology is now also gaining ground in passenger cars. However, it generates additional costs, so that diesel vehicles equipped with it are currently more expensive than cars with gasoline engines. However, since the urea-water solution is produced by around 150 manufacturers worldwide, its availability is guaranteed in the future even if sales increase.
The VDA has determined in a study that a total of around 20 million diesel vehicles will be on the road in Europe with AdBlue technology from 2020 onwards. Therefore, AdBlue dispensers for cars are currently built at gas stations throughout Europe.